1. Field of the Invention
The present invention relates to a battery electrolyte that can be used for the electrolyte of a battery, and a non-aqueous electrolytic secondary battery that can be used as a battery for electric cars and portable electronic devices.
2. Description of the Related Art
There is a need for technology that uses electrical power more effectively in consideration of energy conservation and environmental issues. In order to respond to this need, a means of storing electricity is required that is able to store large amounts of electricity and provide that stored electricity efficiently. Secondary batteries having a large discharge capacity and high discharge voltage while also being able to be repeatedly charged and discharged are optimum for use as such a means of storing electricity.
Lithium secondary batteries are one example of this type of secondary battery. In a lithium secondary battery, a charging reaction occurs during charging in which lithium ions are released from the positive electrode and occluded at the negative electrode during charging, while during discharging, a discharging reaction occurs in which lithium ions are released from the negative electrode and occluded at the positive electrode. In lithium secondary batteries, since both energy density and output density are high, a large discharge capacity and discharge voltage are obtained. In addition, lithium ion secondary batteries, in which a negative electrode active material composed of a carbon material is used for the negative electrode, are expected to be used for portable electronic devices, electric cars and other applications due to their long service life and excellent practicality.
In lithium secondary batteries, an electrolyte made by dissolving a supporting electrolytic (electrolytes salts) salt in an organic solvent is used as the electrolyte. The electrolyte of this type of non-aqueous electrolytic secondary battery may be made by dissolving a supporting electrolyte such as lithium hexafluorophosphate (LiPF6) in an organic solvent of a cyclic carbonate compound such as ethylene carbonate or propylene carbonate. Since cyclic carbonate compounds have a high dielectric constant, the energy density and output density of the battery can be made to be extremely high.
However, due to the high viscosity of cyclic carbonate compounds, the mobility of the lithium ions is low. Consequently, organic solvent in which low-viscosity linear-carbonate compounds such as dimethylcarbonate and diethylcarbonate are mixed into these cyclic carbonate compounds are widely used. However, these linear-carbonate compounds have the shortcomings of being low molecular weight compounds which makes them susceptible to volatilization.
In addition, although attempts have been made to add low molecular weight ether compounds to electrolyte for the purpose of lowering the viscosity of the electrolyte, not only do they have the disadvantage of lowering cycle characteristics, but also end up lowering incombustibility, thereby causing problems in terms of safety.
Therefore, Japanese Unexamined Patent Publication (Kokai) No. 7-282849 discloses the improvement of shelf life and cycle characteristics while also reducing volatility by containing an alkylene bis-carbonate compound in an electrolyte. However, simply containing an alkylene bis-carbonate compound alone makes it difficult to sufficiently increase the incombustibility of the electrolyte. Thus, although this electrolyte has the required safety, that safety is not adequate.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 4-184870 and Japanese Unexamined Patent Publication (Kokai) No. 8-111238 disclose the improvement of safety by using an ester phosphate like a linear alkyl phosphate or cyclic phosphate, and an organic solvent such as a halogen compound.
However, when phosphate ester is used for the primary solvent, a side reaction occurs at the negative electrode interface during charging causing the occlusion of lithium ions to not proceed efficiently. As the result, the battery performance, such as the energy density and the charging and discharging efficiency often decrease significantly.
On the other hand, when ethylene carbonate is used for the primary solvent and a small amount of phosphate ester is added, although the battery performance is not affected and has the required safety, the battery has the problem that the safety decreases. Thus, it was very difficult to reconcile the performance and the safety of the lithium secondary battery.
In addition, there have been no secondary battery that include phosphate esters or halogen compounds and that exhibit an excellent battery performance at temperatures outside the room temperature.
The present invention has been attained in view of the above circumstances. An object of the present invention is to provide an electrolyte that can make the battery performance such as the energy density and output density excellent, can highly maintain the performance at temperatures outside room temperature, and is excellent in incombustibility.
Another object of the present invention is to provide a non-aqueous electrolyte, and a non-aqueous electrolytic secondary battery that has an excellent battery performance such as energy density and the charging and discharging efficiency, cycle characteristic, etc., can maintain the battery performance at temperatures outside room temperature, and also is excellent in safety.
Another object of the present is to provide a non-aqueous electrolyte, and a non-aqueous electrolytic secondary battery in which it is used, that satisfies both high battery performance and safety in good balance as a result of examining the composition, mixing ratio and so forth of a non-aqueous electrolyte solvent that retains the battery characteristics of secondary batteries and has a high level of safety.
A first aspect of the present invention is a battery electrolyte wherein a supporting electrolyte is dissolved in an organic solvent, characterized in that said organic solvent contains (a) a cyclic carbonate compound (b) at least one type of alkyl mono-carbonate compounds represented by chemical formula (1) and alkylene bis-carbonate compounds represented by chemical formula (2), and (c) a phosphorous-containing organic compound:
R1OC(xe2x95x90O)OR2 xe2x80x83xe2x80x83(1) 
wherein, substitution groups R1 and R2 represent identical or different alkyl groups, and at least one of these has at least three carbon atoms; and:
R3OC(xe2x95x90O)OR4OC(xe2x95x90)OR5 
wherein, substitution groups R3 and R5 represent identical or different alkyl groups and have 1 to 4 carbon atoms, and R4 represents a straight chain or branched alkylene group having 1 to 3 carbon atoms.
Cyclic carbonate compounds are able to increase the dielectric constant and so forth of electrolytes. Consequently, they can be made to provide excellent battery characteristics such as being able to increase the energy density of the battery.
The alkyl mono-carbonate compound represented by chemical formula (1) is able to lower the viscosity of an electrolyte. Consequently, since this compound is able to increase the mobility of electrolyte ions and so forth, they can be made to provide excellent battery characteristics such as energy density and output density. In particular, this compound is able to enhance electrolyte performance at low temperatures since it is able to maintain the energy density of batteries at high levels even at low temperatures.
The alkylene bis-carbonate compound represented by chemical formula (2) is able to give the electrolyte excellent storage properties. This alkylene bis-carbonate compound is able to enhance electrolyte performance at high temperatures since it is able to give the electrolyte excellent storage properties at high temperatures, in particular.
The phosphorous-containing organic compound is able to increase the incombustibility of the electrolyte. Consequently, it is able to enhance the safety of the electrolytic salt.
Each of the above compounds are able to effectively demonstrate their functions without impairing the functions of the other compounds. Consequently, the battery electrolyte of the present aspect is able to demonstrate excellent battery performance with respect to energy density, output density and so forth, and, in addition to the battery performance being able to be maintained at a high level even at temperatures other than room temperature, has excellent incombustibility. A battery electrolyte containing the alkylene bis-carbonate compound represented by chemical formula (2) in particular has excellent volatility and storage properties.
Thus, according to the battery electrolyte of the present invention, a battery can be obtained having excellent battery performance, the battery performance is maintained at a high level even at temperatures other than room temperature, and it is extremely safe.
A second aspect of the present invention is a non-aqueous electrolytic secondary battery equipped with a positive electrode and negative electrode that allows release and occlusion of lithium ions, and an electrolyte juxtapositioned between said positive and negative electrodes that is made by dissolving a supporting electrolyte in an organic solvent, characterized in that said organic solvent contains (a) a cyclic carbonate compound, (b) at least one type of alkyl mono-carbonate compounds represented by the above-mentioned chemical formula (1) and alkylene bis-carbonate compounds represented by the above-mentioned chemical formula (2), and (c) a phosphorous-containing organic compound. It is preferable that said organic solvent contains both compounds represented by chemical formulas (1) and (2) as component (b).
In the non-aqueous electrolytic secondary battery of the present aspect, since the electrolyte used has a high dielectric constant, it has extremely good battery performance such as energy density, output density, charging and discharging efficiency and cycle characteristics. In addition, since that electrolyte performance is maintained at a high level at temperatures other than room temperature, battery performance is maintained at a high level even if used in a temperature environment other than room temperature. Since it has excellent incombustibility, the battery is also extremely safe.
Consequently, the non-aqueous electrolytic secondary battery of the present aspect has excellent battery performance such as energy density, output density, charging and discharging efficiency and cycle characteristics, and, in addition to that, the battery performance is able to be maintained not only at room temperature, but also, either in low temperature or high temperature environments, it is extremely safe. Since a non-aqueous electrolytic secondary battery, in which an electrolyte containing the alkylene bis-carbonate compound represented by chemical formula (2) in particular is used, is able to maintain a high level of battery performance even after long-term use since the electrolyte has very low volatility and good storage properties.
Thus, according to the non-aqueous electrolytic secondary battery of the present invention, battery reliability is extremely high since, in addition to being able to power portable electronic devices, electric cars and so forth with advanced functions, it can also be used with peace of mind regardless of the temperature of the environment.
As a result of additional studies to further improve the balance between battery performance and safety after inventing the above-mentioned first and second aspects, when the inventors of the present invention examined a non-aqueous electrolyte in which a cyclic carbonate compound, an alkylene bis-carbonate compound, an alkylene mono-carbonate compound, a glycol diether compound and a phosphorous-containing organic compound were mixed for the above-mentioned organic solvent as a non-aqueous for batteries in which supporting electrolyte is dissolved in organic solvent, it was found that a secondary battery is obtained having even better battery performance such as output characteristics and cycle characteristics while still maintaining safety, thereby leading to completion of a third aspect of the present invention.
The non-aqueous electrolyte of this third aspect is a non-aqueous electrolyte for batteries in which an electrolyte salt is dissolved in an organic solvent characterized by the organic solvent containing at least one type each of a cyclic carbonate compound, an alkylene bis-carbonate compound represented by the above-mentioned chemical formula (2), an alkyl mono-carbonate compound represented by the above-mentioned chemical formula (1), a glycol diether compound represented by chemical formula (3) below and a phosphorous-containing organic compound:
R6Oxe2x80x94(R7O)nxe2x80x94R8 xe2x80x83xe2x80x83(3) 
wherein, substitution groups R6 and R8 are alkyl groups having 1 to 8 carbon atoms that may or may not be substituted with a halogen atom, substitution group R7 is an alkylene group having 2 to 4 carbon atoms that may or may not be substituted with a halogen atom, and n is 1xe2x89xa6nxe2x89xa64, provided that at least one of the substitution groups R6, R7 and R8 is substituted with a halogen atom.
It is preferable that 5 to 35 vol % of each of the alkylene bis-carbonate compound represented by the above-mentioned chemical formula (2), the glycol diether compound represented by chemical formula (3) and the phosphorous-containing organic compound be contained assuming the total amount of the above-mentioned organic solvent to be 100 vol %.
It is preferable that the above-mentioned cyclic carbonate compound be contained at 20 to 50 vol % assuming the total amount of the above-mentioned organic solvent to be 100 vol %.
It is preferable that the alkyl mono-carbonate compound represented by the above-mentioned chemical formula (1) be contained at 30 to 50 vol % assuming the total amount of the above-mentioned organic solvent to be 100 vol %.
It is preferable that the above-mentioned phosphorous-containing organic compound contain at least one type of organic compound selected from the group consisting of phosphate esters, phosphonate esters or phosphinate esters.
It is preferable that the above-mentioned electrolyte be composed of one type or a combination of at least two or more types of salts selected from the group consisting of inorganic salt composed of lithium ion and an anion selected from among PF6xe2x88x92, BF4xe2x88x92, ClO4xe2x88x92 and AsF6xe2x88x92, and an organic salt composed of a lithium ion and an anion selected from among SO3CF3xe2x88x92, N(CF3SO2)2xe2x88x92, C(CF3SO2)3xe2x88x92 and their derivatives.
A fourth aspect of the present invention is a non-aqueous electrolytic secondary battery having a non-aqueous electrolyte, positive electrode and negative electrode, characterized by the use of the non-aqueous electrolyte of the above-mentioned third aspect containing at least one type each of a cyclic carbonate compound, an alkylene bis-carbonate compound represented by chemical formula (2), an alkyl mono-carbonate compound represented by chemical formula (1), a glycol diether compound represented by chemical formula (3) and a phosphorous-containing organic compound as an organic solvent of a non-aqueous electrolyte.
A fifth aspect of the present invention is a non-aqueous electrolyte in which an electrolytic salt salt is dissolved in said organic solvent characterized in that the organic solvent contains at least one type of glycol diether selected from the glycol diethers represented by chemical formula (4) below:
R9Oxe2x80x94(R10O)nxe2x80x94R11 xe2x80x83xe2x80x83(4) 
wherein, R9 and R11 are alkyl groups having 1 to 8 carbon atoms that may or may not be substituted with a halogen atom, R10 is an alkylene group having 1 to 8 carbon atoms that may or may not be substituted with a halogen group provided that at least one of R9, R10 and R11 is substituted with a halogen atom, and n is 1xe2x89xa6nxe2x89xa64.
The use of at least one type of glycol diether represented by chemical formula (4) is able to yield satisfactory output characteristics by lowering the internal resistance of the battery as a result of increasing the mobility of lithium ions at the solid-liquid interface. The mechanism by which the addition of glycol diether represented by chemical formula (4) to non-aqueous electrolyte increases the mobility of lithium ions at the solid-liquid interface is thought to be the result of hydrogen atoms of the glycol diether represented by chemical formula (4) demonstrating a surface activator-like effect particularly as a result of being substituted by fluorine atoms, thereby increasing the affinity for the electrode of the non-aqueous electrolyte.
In addition, those glycol diether compounds represented by chemical formula (4) in which R9 and R11 are alkyl groups having no more than 3 carbon atoms, R10 is an alkylene group having no more than 2 carbon atoms, and n is 2 or less are particularly preferable due to the excellent solubility of the electrolyte. Compounds in which R11 is an alkyl group substituted with a fluorine atom are preferable due the large effect of lowering internal resistance. Although the details of this reason are unknown, it is thought that the ion-delivery and acceptance barrier on the electrode surface is lowered due to improved wettability of the electrode-electrolyte interface resulting from enhancement of the surface activator-like effect mentioned above.
Thus, use of the non-aqueous electrolyte of the present invention in a non-aqueous electrolytic battery makes it possible to improve the output characteristics and so forth of the battery.
Moreover, it is preferable that the above-mentioned organic solvent contains at least one type of compound selected from among, for example, carbonates, lactones, ethers, sulfolanes and dioxolanes. The use of a non-aqueous electrolyte to which has been added a substance that is able to increase the solubility of an electrolyte salt and the dielectric constant and viscosity of the electrolyte, like a carbonate, lactone, ether, sulfolane and dioxolane makes it possible to further enhance the performance of the battery.
Moreover, it is also preferable that the above-mentioned organic solvent contain at least one type of compound selected from among, for example, phosphate ester, phosphonate ester and phosphinate ester. The use of a non-aqueous electrolyte to which has been added a substance like phosphate ester, phosphonate ester and phosphinate ester that is able to improve the incombustibility of the non-aqueous electrolyte makes it possible to improve the safety of a non-aqueous electrolytic battery.
In addition, it is preferable that the above-mentioned electrolyte salt be at least one type of inorganic salt selected from the group consisting of, for example, LiPF6, LiBF4, LiClO4 and LiAsF6, organic salt selected from the group consisting of, for example, LiSO3CF3, LiN(CF3SO2)2 and LiC(CF3SO2)3, and derivatives of said organic salts. These electrolyte salts are able to enhance the properties of the electrolyte. For this reason, the use of an incombustible electrolyte in which these electrolyte salts are dissolved is able to further improve the performance of the battery.
A sixth aspect of the present invention is characterized by being a non-aqueous electrolytic secondary battery provided with the non-aqueous electrolyte of the above-mentioned fifth aspect. The non-aqueous electrolytic secondary battery of the present aspect has excellent output characteristics for the reasons previously described as a result of having a non-aqueous electrolyte containing at least one type of glycol diether selected from among the glycol diethers represented by chemical formula (4).
Thus, the use of the non-aqueous electrolytic secondary battery of the present invention for the battery of a portable electronic device, automobile or other application is able to drive those applications with satisfactory performance and satisfactory reliability.